This invention relates to heat recovery steam generators (HRSG), and more particular to a duct burner assembly for a liner of a HRSG.
Natural gas and to a lesser extent fuel oil are the sources of much of the electrical energy consumed today. Combined cycle power plants convert those fuels into electrical energy in a highly efficient manner. There are three major components in a combined cycle power plant: a combustion turbine with electrical generator, a Heat Recovery Steam Generator, and a steam turbine with electrical generator. Basically, the fuel, whether it is natural gas or oil, burns within the combustion turbine, and the turbine drives an electrical generator that produces some of the electrical energy furnished by the plant. The combustion turbine also discharges exhaust gas at elevated temperatures often exceeding 1000° F. The exhaust gas flows through the HRSG which extracts heat from it to convert subcooled water into superheated steam that flows into the steam turbine, which in turn drives another electrical generator that produces more electricity.
Duct burners use supplementary firing to increase the heat energy of a gas turbine's exhaust, making it possible to increase the output of a downstream heat-recovery steam generator. Using a HRSG with auxiliary or supplemental fuel firing in a duct burner can increase steam production, control steam superheat temperature, or meet process steam requirements. HRSG designs can also directly incorporate selective catalytic reduction (SCR) technology for nitrous oxide control.
A common problem for duct burners with heavy supplemental firing is overheating and deterioration of the liners of a combustion chamber. Thus, reliable control of the liner temperature regime is very important to prevent deterioration. This is especially true for the modern generation of combustion turbines and liquid fuels, such as oil or kerosene.
Duct burners include burner sections that produce high flame temperatures including significant thermal radiation. Duct liners are used to confine and protect ceramic fiber insulation behind the liners and the HRSG outer casing. In some cases, liners are unable to withstand the elevated temperatures over extended periods of operation. The liners fail, and when they do, the ducting that they are designed to protect is damaged.
The turbine exhaust gas approaching the burner in the ducting of a HRSG, while being at an elevated temperature, is considerably cooler than the flames produced at the burner. Turbine exhaust flowing along the liners in the combustion chamber is not heated directly by the fuel combustion. As liners absorb radiant energy from the flames, they are cooled convectively by the adjacent turbine exhaust. The amount of flow along the liner and the degree of mixing of this flow with the bulk flow heated by the flame will affect the convective heat transfer from the liner.
In existing duct burners, there are no special elements which could properly form a cold or cooler gas flow over the liners. Moreover, the burner pipe and guide tube in the gap between the duct burner framework and liner generate turbulence. Additionally, some elements of the duct burner, such as flame stabilizers and gas baffles, generate strong turbulence in the gas flow. These elements together with turbulent turbine exhaust flow destroy the protective cold film over the liners. As a result, the turbulent flow decreases the heat transfer coefficient from the liner to the coolant and can increase the temperature of the liner in the vicinity of the flame. A less turbulent flow would increase the heat transfer coefficient from the liner to the coolant and lower the temperature of the liner.
Therefore, there is a need for effective cooling of the liner of a HRSG duct burner with less turbulence of turbine exhaust flow.
Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings.